Amplitude modulation controller for polar transmitter

ABSTRACT

Apparatus for generating a modulation signal for use in modulating the power supply of a power amplifier uses coarse and fine control for controlling the amplitude of the modulation signal, and thereby controlling the output power of the power amplifier. The modulation signal may be generated in the digital domain and converted to the analog domain by a digital-to-analog converter, with the digital-to-analog converter providing the fine control and a variable gain amplifier providing the coarse control of the analog signal.

BACKGROUND

1. Technical Field

The disclosure relates to apparatus for controlling a modulation signalfor use in modulating the power supply of a power amplifier, a modulecomprising the apparatus, a transmitter comprising the module, andcommunications equipment comprising the transmitter.

2. Description of the Related Art

Polar modulation architecture has become quite popular in recent years,at least in scientific publications, as an alternative to the directCartesian (I&Q) up-conversion architecture, for transmitters targetinghand-set applications where communication standards with amplitudemodulation, such as EDGE, are used. This is because a polar modulationtransmitter can have a lower power consumption and lower cost. The costis lower, at least in part, because there is no need for a surfaceacoustic wave (SAW) filter at the output of the transmitter.

A typical polar modulation transmitter 100 of the prior art isillustrated in FIG. 1. Separate amplitude and frequency components ofthe modulation signal are generated in the digital domain and providedto, respectively, inputs 110 and 120. A sigma-delta converter 140converts the digital frequency component to a digital control signal forcontrolling the divider ratio of a fractional-N phase locked loop 150for modulating the phase of a voltage controlled oscillator 155. Theoutput of the voltage controlled oscillator 155 is coupled to asaturated power amplifier (PA) 160, and the amplified signal is providedat an output 130 to an antenna. The digital amplitude component isprovided to an amplitude modulation controller 170 which converts thedigital amplitude component to the analog domain and scales it based ona scaling signal at an input 175 to provide an analog amplitudemodulation signal to a power supply modulator 180 for modulating thesupply voltage of the PA 160. This supply voltage determines the powerof the amplified signal output by the PA 160 and hence to the antenna.Typically, the power supply modulator 180 and the PA 160 are provided ina front-end module (FEM) 190.

Typically, the PA 160 needs a certain DC voltage to start providing anysignal at the output 130 to the antenna. This is illustrated in FIG. 2by a typical graph of PA output amplitude as a function of PA supplyvoltage, where it can be seen that almost no output power is generateduntil the supply voltage reaches about 0.2V. So, the amplitudemodulation controller 170 usually receives a DC offset control signal atan input 185 for controlling DC offset in the analog amplitudemodulation signal.

The desired power of the signal at the antenna depends on the length andquality of the signal path between the transmitter 100 (in a handset)and a base station. These parameters can vary greatly, meaning that thePA 160 preferably provides the amplified signal over a large range ofpower levels, typically about three power decades.

If the scaling is performed in the digital domain, the amplitudemodulation controller 170 usually requires a digital-to-analog converter(DAC) having a high dynamic range in order to provide an analogamplitude modulation signal having a good signal-to-quantization-noiseratio and suitable for allowing the power supply modulator 180 tomodulate the supply voltage to the PA 160 sufficiently for the PA 160 tooutput the amplified signal over the desired large range of powerlevels. However, a typical high dynamic range DAC having over 14-bitsand using a sampling frequency of at least 3.25 MHz requires a largeoperating current and a large chip area in an integrated circuit.Likewise, if the scaling is performed in the analog domain, prior artdevices employ complex analog circuit elements to obtain fine control ofthe gain applied to the analog signal over a wide range of gain values.

BRIEF SUMMARY

According to a first aspect of the disclosure there is provided anapparatus for controlling a modulation signal for use in modulating apower supply of a power amplifier, comprising:

a power controller for generating an indication of a desired mean outputpower of the power amplifier, the indication comprising a coarse controlsignal providing a relatively coarse indication and a fine controlsignal providing a relatively fine indication;

a coarse amplitude controller for providing relatively coarse control ofthe amplitude of the modulation signal in response to the coarse controlsignal; and

a fine amplitude controller for providing relatively fine control of theamplitude of the modulation signal in response to the fine controlsignal.

Thus the disclosure provides an improved architecture for an amplitudemodulation controller by providing separate coarse and fine control ofthe amplitude of the modulation signal. By providing separate coarse andfine control, fine control of the amplitude of the modulation signalover a wide range of amplitudes can be achieved with a lower dynamicrange DAC and/or less complex analog circuitry elements than in theprior art. This, in turn, allows lower power consumption and small chiparea to be achieved. Furthermore, the requirement for wide dynamic rangefiltering associated with use of a high dynamic range DAC can beavoided, further reducing power consumption and chip area.

Optionally, the modulation signal is generated as a digital modulationsignal, and the apparatus comprises a DAC for converting the digitalmodulation signal to an analog modulation signal, wherein the DAC isadapted to provide the relatively fine control of the amplitude of theanalog modulation signal in response to the fine control signal. Thusthe fine control can be integrated into the DAC, enabling highresolution control with low complexity, low power consumption, and smallchip area.

Optionally, the DAC may comprise an array of selectable current elementseach adapted to provide a reference current to either output of adifferential pair of outputs, a selector for selecting, dependent on thedigital modulation signal, which of the current elements provide theirrespective reference current to the outputs, and a current controllerfor controlling the reference current dependent on the fine controlsignal. Thus the fine control can be implemented simply by varying thereference current, and can be integrated into the DAC in a simplemanner, enabling high resolution control with low complexity, low powerconsumption, and small chip area.

Optionally, the apparatus may comprise a semiconductor device forproviding a bandgap voltage, wherein the current controller comprises aprogrammable resistance responsive to the fine control signal forgenerating the reference current from the bandgap voltage. The use of abandgap voltage contributes to a highly accurate reference current.

Optionally, the DAC is adapted to scale the amplitude of the analogmodulation signal in response to the fine control signal whilst theamplitude range of the digital modulation signal remains substantiallyconstant. Thus the amplitude range of the modulation may be controlledsolely in the analog domain, which can avoid the need for low amplitudedigital signals which would have a relatively poorsignal-to-quantization-noise ratio. A constant amplitude digitalmodulation signal has a constant signal-to-quantization-noise ratio.

Optionally, the DAC is arranged such that the digital modulation signaloccupies at least 90%, and preferably 100%, of the digital range of theDAC, irrespective of the required mean output power indicated by thepower controller. By using the majority, or the whole, of the DACdigital range, even when a low output power level is desired, a highsignal-to-quantization-noise ratio can be maintained. Conversely, for aparticular signal-to-quantization-noise ratio, the word size of the DACmay be reduced, enabling low complexity, low power consumption, andsmall chip area. For example, the signal-to-quantization-noise ratio isthe same for a 13-bit DAC operated over 11.25% of its range, a 12-bitDAC operated over 22.5% of its range, an 11-bit DAC operated over 45% ofits range, and a 10-bit DAC operated over 90% of its range.

Optionally, the coarse amplitude controller comprises a variable gainamplifier having a gain responsive to the coarse control signal. Such acontroller can have a low complexity, low power consumption, and smallchip area, avoiding for example the need for a more complex analogmultiplier circuit.

Optionally, the variable gain amplifier is adapted to control the DCoffset of the analog modulation signal in response to a DC controlsignal. Thus DC offset control can be combined with amplitude control.

Optionally, the DAC is adapted to provide the analog modulation signalas a differential signal, and the coarse amplitude controller is adaptedto provide differential to single ended conversion of the analogmodulation signal. Thus the amplitude control may be performed ondifferential signals which are inherently resistant to substrate noiseand interferers, and conversion to single ended signals as required formodulating the power supply of a power amplifier can be combined withthe amplitude control.

The disclosure also provides a module comprising the apparatus accordingto the first aspect of the disclosure, an amplifier, and a power supplymodulator for modulating a power supply to the amplifier with the analogmodulation signal. The disclosure also provides a transmitter comprisingthe module, and a communication equipment comprising the transmitter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will now be described, by way of example only, withreference to the accompanying drawings where:

FIG. 1 is block schematic diagram of a polar modulation transmitter ofthe prior art;

FIG. 2 is a graph of power amplifier output power as a function of poweramplifier supply voltage for the polar modulation transmitter shown inFIG. 1;

FIG. 3 is a block schematic diagram of an apparatus for controlling amodulation signal according to a preferred embodiment of the presentdisclosure; and

FIG. 4 is a table of DAC currents and voltages for different digitalinput values.

FIG. 5 is a block diagram of a communication device according to oneembodiment of the present disclosure.

DETAILED DESCRIPTION

In a preferred embodiment of the present disclosure, the amplitudemodulation controller 170 of the polar modulation transmitter 100 of theprior art shown in FIG. 1 is replaced with the amplitude modulationcontroller 270 shown in FIG. 3. As in the prior art, the amplitudemodulation controller 270 of the preferred embodiment of the presentdisclosure receives the digital amplitude modulation signal at an input110. It also receives the scaling signal at the input 175 and DC offsetcontrol signal at the input 185 and, at an output 46, provides a scaledamplitude modulation signal with appropriate DC offset according tothese signals. However, in contrast to the prior art, the amplitudemodulation controller 270 of the preferred embodiment of the presentdisclosure has a power control stage 200 for controlling scaling of theamplitude modulation signal based on the scaling signal at the input 175using a fine control and a coarse control, as described in more detailbelow.

Referring to FIG. 3, the digital amplitude modulation signal is providedat the input 110 of the amplitude modulation controller 270 which iscoupled to a DAC 20. The DAC 20 is a current DAC and its output currentI_(P), I_(N), and output voltage V_(P), V_(N), are dependent on areference current I_(ref) provided by a reference current generator 24,as well as being dependent on resistors R₀ coupled between output nodes26, 28 and a voltage rail V_(DD). An output of the DAC 20 is coupled toan input of a low pass reconstruction filter 30 for smoothing the signalprovided at the output of the DAC 20, by removing replicas of thespectrum of the digital amplitude modulation signal which occur atmultiples of the sampling frequency of the digital amplitude modulationsignal. An output of the reconstruction filter 30 is coupled to an inputof the variable gain amplifier 40 which amplifies the smoothed signaland provides the analog amplitude modulation signal at the output 46.

Looking firstly at the fine control of the scaling of the amplitudemodulation signal, a band-gap circuit 70 generates a reference voltageV_(bg) which is independent of temperature and integration process. Theoutput impedance of the band-gap circuit 70 should be much lower thanvariable resistors R3 and R4, described later. An output of the band-gapcircuit 70 is coupled to the reference current generator 24, which usesthe reference voltage V_(bg) to generate the reference current I_(ref).

The reference current generator 24 is programmable, such that themagnitude of the reference current I_(ref) is dependent on a digitalvalue provided at an input 50 by the power controller 200 based on thescaling signal at the input 175. This digital value comprises a finecontrol signal. So, the reference current I_(ref) is programmable inrelatively small steps over a relatively small range. The referencecurrent generator 24 may employ, for example, a programmable resistance27 to derive the reference current I_(ref) from the reference voltageV_(bg). Since a multiple of the reference current I_(ref) flows throughthe same type of resistor in the subsequent stages of the amplitudemodulation controller 270, the output signal accuracy is given by theabsolute accuracy of the band-gap voltage and matching of components.

The DAC 20 has a differential output and comprises an array ofselectable current elements 23 each of which can provide a currentI_(ref) to a positive output node 26 of the DAC 20, their sumconstituting the current I_(P), or to a negative output node 28 of theDAC 20, their sum constituting the current I_(N). The selection ofcurrent elements is made by a selector 25 dependent on the digital valueof the digital amplitude modulation signal at the input 110. Thisenables monotonic and highly linear digital-to-analog conversion.

The table of FIG. 4 provides an example of the relationship between thedigital value of the digital amplitude modulation signal (first column)and the current I_(P) at the positive output node 26 (second column) andthe current I_(N) at the negative output node 28 (third column), wherethe current values in the table are expressed normalized to the value ofI_(ref), i.e., the values indicate a multiple of I_(ref). The fourthcolumn in FIG. 4 indicates a differential voltage V_(P)-V_(N), whereV_(p) and V_(N) are the voltages at respectively the positive outputnode 26 and the negative output node 28 resulting from passing theoutput currents I_(P), I_(N) through respective resistors R₀. Forexample, a digital value of 0 results in I_(P)=I_(ref).n andI_(N)=I_(ref).n such that the differential output voltage V_(P)−V_(N)=0.As another example, a digital value of (n−1)/2 results inI_(P)=I_(ref).(n+1)/2 and I_(N)=I_(ref).(3n−1)/2 such that thedifferential output voltage V_(P)−V_(N)=R₀.I_(ref).(n−1). For a 10-bitDAC, n=1023. After filtering the voltages V_(P) and V_(N), thereconstruction filter 30 provides the filtered voltages respectivelyV_(P2) and V_(N2) to differential inputs of the variable gain amplifier40.

Looking now at the coarse control of the scaling of the amplitudemodulation signal, the variable gain amplifier 40 comprises anoperational amplifier 42 which provides differential to single-endedconversion, and a gain control stage 44. The gain control stage 44comprises a variable resistor R₁ coupled in series with each of thedifferential inputs of the variable gain amplifier 40 and the respectivedifferential inputs of the operational amplifier 42, a feedback circuitcoupled between the output 46 and a negative input of the operationalamplifier 42, and a DC-setting circuit coupled between a positive inputof the operational amplifier 42 and a DC control input 48 of thevariable gain amplifier 40. The feedback circuit comprises a capacitorC₂ coupled in parallel with a variable resistor R₂. The DC-settingcircuit comprises a capacitor C₂ coupled between the positive input ofthe operational amplifier 42 and ground, and a variable resistor R₂coupled in series between the positive input of the operationalamplifier 42 and the DC control input 48. This combination C₂, R₂provides a first order low pass filter which can assist in meetinglow-noise specifications of the transmitted spectrum, especially forEDGE where spectral components over 20 MHz can fall into the receivefrequency band and degrade reception by a nearby handset. Similarly, fora transceiver complying with the UMTS standard, it is important tominimize out-of-band spectral components in a transmitted signal asthese can degrade simultaneous reception during duplex operation of ahandset. The gain of the variable gain amplifier 40 is controlled byvarying the value of the variable resistors R₁, R₂ in response to adigital value applied at a gain control input 90 by the power controller200. This digital value comprises a coarse control signal. C₂ is variedin a corresponding manner to keep the product R₂C₂ constant.

The digital value provided by the power controller 200 at the input 50for programming the magnitude of the reference current I_(ref) used bythe DAC 20, and the digital value applied by the power controller 200 atthe gain control input 90 to control the gain of the variable gainamplifier 40, are such that the overall gain of the modulation signal iscontrolled by the scaling signal at the input 175. However, as finecontrol of the scaling is provided by varying the reference currentI_(ref) of the DAC 20 and coarse control of the scaling is provided byvarying the gain of the variable gain amplifier 40, fine control isprovided over a wide range with less complexity. For example, 10 finecontrol settings and 10 coarse control settings can achieve 100different gain levels using just 20 control settings overall. If 100different gain levels were to be provided in a single stage of gaincontrol, as in the prior art, 100 gain control settings would be neededand the complexity of the gain control would be approximately five timeshigher.

In this embodiment, the power controller 200 is implemented in aprocessor, although suitable circuits or multiple processors can be usedin other embodiments. Also, the digital amplitude modulation signal isreceived at the DAC 20 independently of the power controller 200.However, in other embodiments, a processor in which the power controller200 is implemented may also generate the digital amplitude modulationsignal. The DAC 20 may therefore receive the digital amplitudemodulation signal from the processor.

The DC offset of the signal at the output 46 is controlled by varying aDC offset voltage V_(offset) voltage applied at the DC control input 48.In more detail, a DC offset generator 80 is coupled to the band-gapcircuit 70. The DC offset generator 80 has a voltage divider, comprisingseries coupled variable resistors R₃ and R₄, to which the band-gapvoltage V_(bg) is applied. The divided voltage is applied to an input ofan amplifier 82, the output of which is coupled to the DC control input48 of the variable gain amplifier 40 and provides the DC offset voltageV_(offset). A capacitor C is also coupled between the input of theamplifier 82 and ground in order to reduce noise, for example thermalnoise from resistors, and interference.

The DC offset voltage V_(offset) is less than the band gap voltageV_(bg). It is controlled by varying the values of at least one of thevariable resistors R₃ and R₄ of the voltage divider in response to theDC offset control signal at the input 185. In this way, the DC offset atthe output 46 of the variable gain amplifier 40 can be adjusted tocompensate for any unwanted offset in the modulation signal and toprovide a wanted offset voltage to the output 46 to ensure an RF outputsignal, as described above.

A feature of the amplitude modulation controller 270 of the preferredembodiment of the present disclosure is that the analog amplitudemodulation voltage provided at the output 46 is single ended and alwayspositive. This is in contrast to a Cartesian architecture in which I andQ signals have both polarities. Furthermore, the amplitude of the analogamplitude modulation signal can be varied over a wide range. This is incontrast to the scaling in a transmitter using a Cartesian architecture,where the scaling is normally done in a mixer or in a variable gainpre-amplifier preceding a linear power amplifier.

Furthermore, by scaling the amplitude modulation signal in the analogdomain, the DAC 20 is not required to have a relatively high dynamicrange and the reconstruction filter 30 is not required to have a widedynamic range, in contrast to the prior art. Indeed, as the digitalmodulation signal is not scaled according to the preferred embodiment ofthe disclosure, its amplitude range remains substantially constant. Thedigital range of the DAC 20 can therefore be conveniently matched to theamplitude range of the digital amplitude modulation signal. In otherwords, even if the scaled analog amplitude modulation signal is notrequired to be more than say 10% of the maximum amplitude, the amplituderange of the digital amplitude modulation signal can still occupy, say,70% or 80% or 90% or 100% of the digital range of the DAC 20.

Furthermore, the separate coarse and fine control also keeps thecomplexity low. If scaling were to be provided in the analog domainsolely by the DAC 20, the DAC 20 would require a higher complexity,higher power and larger chip area. More significantly, thereconstruction filter 30 would need a wide dynamic range, resulting inincreased power consumption, and chip area would be larger due to therequirement for many gain stages and due to the requirement forincreased transistor sizes to reduce noise. Similarly, if gain controlwere to be provided in the analog domain solely by the variable gainamplifier 40, such an amplifier would require a higher complexity,higher power and larger chip area, such as a large array of selectableresistors for gain setting.

Although embodiments have been described in which the current providedby each of the selectable current elements 23 of the DAC 20 is the sameas the reference current I_(ref) provided by the current generator 24,in practice the current provided by the current elements 23 of the DACmay be a scaled version of the reference current provided by the currentgenerator 24, for example 2I_(ref) or I_(ref)/2.

Although embodiments have been described which use a current DAC 20, theuse of other types of DAC, for example a resistor ladder DAC, is notprecluded.

FIG. 5 is a block diagram of a communication device 300, such as amobile phone, headset, etc. The communication device 300 includes aprocessor 302, controlling the operations of the device 300, and atransmitter 304 configured to transmitted communication signal undercontrol of the processor 302. The transmitter 304 includes an antenna306 and a transmitter circuit 308 that includes the amplitude modulationcontroller 270 of FIG. 3 and the FEM 190, Sigma-Delta converter 140, andPLL 150 from FIG. 1.

This disclosure can be employed in transmitters and transceivers for allcommunication systems using amplitude modulation of a transmittedsignal, such as EDGE, UMTS and the Bluetooth 2.0+EDR standard. Althoughthe disclosure may be used in applications using both amplitude andfrequency or phase modulation, it may also be used in applications whereamplitude modulation is used without frequency or phase modulation.

From reading the present disclosure, other variations and modificationswill be apparent to the skilled person. Such variations andmodifications may involve equivalent and other features which arealready known in the art of transmitter design, and which may be usedinstead of, or in addition to, features already described herein.

Although the appended claims are directed to particular combinations offeatures, it should be understood that the scope of the presentdisclosure also includes any novel feature or any novel combination offeatures disclosed herein either explicitly or implicitly or anygeneralization thereof, whether or not it relates to the same disclosureas presently claimed in any claim and whether or not it mitigates any orall of the same technical problems as does the present disclosure.

Features which are described in the context of separate embodiments mayalso be provided in combination in a single embodiment. Conversely,various features which are, for brevity, described in the context of asingle embodiment, may also be provided separately or in any suitablesub-combination.

The applicant hereby gives notice that new claims may be formulated tosuch features and/or combinations of such features during theprosecution of the present application or of any further applicationderived therefrom.

For the sake of completeness it is also stated that the term“comprising” does not exclude other elements or steps, the term “a” or“an” does not exclude a plurality, a single processor or other unit mayfulfill the functions of several means recited in the claims andreference signs in the claims shall not be construed as limiting thescope of the claims.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. An apparatus for controlling a modulation signal for use inmodulating a power supply of a power amplifier, comprising: a powercontroller configured to generate an indication of a desired mean outputpower of the power amplifier, the indication comprising a coarse controlsignal providing a relatively coarse indication and a fine controlsignal providing a relatively fine indication; a coarse amplitudecontroller configured to provide relatively coarse control of anamplitude of the modulation signal in response to the coarse controlsignal; and a fine amplitude controller configured to provide relativelyfine control of the amplitude of the modulation signal in response tothe fine control signal.
 2. An apparatus as claimed in claim 1, whereinthe coarse amplitude controller comprises a variable gain amplifierhaving a gain responsive to the coarse control signal.
 3. An apparatusas claimed in claim 2, wherein the variable gain amplifier is configuredto control the DC offset of the modulation signal in response to a DCcontrol signal.
 4. An apparatus as claimed in claim 1, wherein the finecontrol signal is a digital fine control signal and the fine amplitudecontroller comprises a digital-to-analog converter configured to convertthe digital fine control signal to an analog fine control signal andprovide the relatively fine control of the amplitude of the analogmodulation signal in response to the digital fine control signal.
 5. Anapparatus as claimed in claim 4, wherein the digital-to-analog convertercomprises: a differential pair of outputs; an array of selectablecurrent elements configured to provide respective reference currents toeither output of the differential pair of outputs; a selector configuredto select, dependent on the digital fine control signal, which of thecurrent elements provide their respective reference current to theoutputs; and a current controller configured to control the selectedreference current dependent on the digital fine control signal.
 6. Anapparatus as claimed in claim 5, comprising a semiconductor bandgapvoltage circuit configured to provide a bandgap voltage, and wherein thecurrent controller comprises a programmable resistance responsive to thedigital fine control signal and configured to generate the referencecurrent from the bandgap voltage.
 7. An apparatus as claimed in claim 4,wherein the digital-to-analog converter is configured to scale theanalog fine control signal in response to the digital fine controlsignal while an amplitude range of the digital fine control signalremains constant.
 8. An apparatus as claimed in claim 4, wherein thedigital-to-analog converter is arranged such that the digital finecontrol signal occupies at least 90% of a digital range of thedigital-to-analog converter, irrespective of the mean output powerindicated by the power controller.
 9. An apparatus as claimed in claim4, wherein the digital-to-analog converter is configured to provide theanalog fine control signal as a differential signal, and the coarseamplitude controller is configured to provide differential to singleended conversion of the analog fine control signal.
 10. A modulecomprising: an amplifier; a power supply modulator for modulating apower supply to the amplifier with an analog modulation signal; and anapparatus configured to provide the analog modulation signal, theapparatus including: a power controller configured to generate anindication of a desired mean output power of the power amplifier, theindication comprising a coarse control signal providing a relativelycoarse indication and a fine control signal providing a relatively fineindication; a coarse amplitude controller configured to providerelatively coarse control of an amplitude of the modulation signal inresponse to the coarse control signal; and a fine amplitude controllerconfigured to provide relatively fine control of the amplitude of themodulation signal in response to the fine control signal.
 11. A moduleas claimed in claim 10, wherein the fine control signal is a digitalfine control signal and the fine amplitude controller comprises adigital-to-analog converter configured to convert the digital finecontrol signal to an analog fine control signal and provide therelatively fine control of the amplitude of the analog modulation signalin response to the digital fine control signal.
 12. A module as claimedin claim 11, wherein the coarse amplitude controller comprises avariable gain amplifier having a gain responsive to the coarse controlsignal.
 13. A module as claimed in claim 11, wherein thedigital-to-analog converter comprises: a differential pair of outputs;an array of selectable current elements configured to provide respectivereference currents to either output of the differential pair of outputs;a selector configured to select, dependent on the digital fine controlsignal, which of the current elements provide their respective referencecurrent to the outputs; and a current controller configured to controlthe selected reference current dependent on the digital fine controlsignal.
 14. A module as claimed in claim 11, wherein thedigital-to-analog converter is configured to scale the analog finecontrol signal in response to the digital fine control signal while anamplitude range of the digital fine control signal remains constant. 15.A module as claimed in claim 11, wherein the digital-to-analog converteris configured to provide the analog fine control signal as adifferential signal, and the coarse amplitude controller is configuredto provide differential to single ended conversion of the analog finecontrol signal.
 16. A transmitter, comprising: an antenna; and a modulethat includes: an amplifier; a power supply modulator for modulating apower supply to the amplifier with an analog modulation signal; and anapparatus configured to provide the analog modulation signal, theapparatus including: a power controller configured to generate anindication of a desired mean output power of the power amplifier, theindication comprising a coarse control signal providing a relativelycoarse indication and a fine control signal providing a relatively fineindication; a coarse amplitude controller configured to providerelatively coarse control of an amplitude of the modulation signal inresponse to the coarse control signal; and a fine amplitude controllerconfigured to provide relatively fine control of the amplitude of themodulation signal in response to the fine control signal.
 17. Atransmitter as claimed in claim 16, further comprising: a sigma-deltaconverter configured to convert a digital frequency component of theanalog modulation signal to a digital control signal; and a phase-lockedloop configured to provide an input signal to the power amplifier basedon the digital control signal.
 18. A transmitter as claimed in claim 16,wherein the fine control signal is a digital fine control signal and thefine amplitude controller comprises a digital-to-analog converterconfigured to convert the digital fine control signal to an analog finecontrol signal and provide the relatively fine control of the amplitudeof the analog modulation signal in response to the digital fine controlsignal.
 19. A transmitter as claimed in claim 18, wherein the coarseamplitude controller comprises a variable gain amplifier having a gainresponsive to the coarse control signal.
 20. A transmitter as claimed inclaim 18, wherein the digital-to-analog converter comprises: adifferential pair of outputs; an array of selectable current elementsconfigured to provide respective reference currents to either output ofthe differential pair of outputs; a selector configured to select,dependent on the digital fine control signal, which of the currentelements provide their respective reference current to the outputs; anda current controller configured to control the selected referencecurrent dependent on the digital fine control signal.
 21. A transmitteras claimed in claim 18, wherein the digital-to-analog converter isconfigured to scale the analog fine control signal in response to thedigital fine control signal while an amplitude range of the digital finecontrol signal remains constant.
 22. A transmitter as claimed in claim18, wherein the digital-to-analog converter is configured to provide theanalog fine control signal as a differential signal, and the coarseamplitude controller is configured to provide differential to singleended conversion of the analog fine control signal.
 23. A communicationdevice, comprising: a communication processor configured to provide amodulation signal; and a transmitter that includes: an antenna; and amodule that includes: an amplifier; a power supply modulator formodulating a power supply to the amplifier with an analog modulationsignal; and an apparatus configured to provide the analog modulationsignal, the apparatus including: a power controller configured togenerate an indication of a desired mean output power of the poweramplifier, the indication comprising a coarse control signal providing arelatively coarse indication and a fine control signal providing arelatively fine indication; a coarse amplitude controller configured toprovide relatively coarse control of an amplitude of the modulationsignal in response to the coarse control signal; and a fine amplitudecontroller configured to provide relatively fine control of theamplitude of the modulation signal in response to the fine controlsignal.
 24. A communication device as claimed in claim 23, furthercomprising: a sigma-delta converter configured to convert a digitalfrequency component of the analog modulation signal to a digital controlsignal; and a phase-locked loop configured to provide an input signal tothe power amplifier based on the digital control signal.
 25. Acommunication device as claimed in claim 23, wherein the fine controlsignal is a digital fine control signal and the fine amplitudecontroller comprises a digital-to-analog converter configured to convertthe digital fine control signal to an analog fine control signal andprovide the relatively fine control of the amplitude of the analogmodulation signal in response to the digital fine control signal.
 26. Acommunication device as claimed in claim 25, wherein the coarseamplitude controller comprises a variable gain amplifier having a gainresponsive to the coarse control signal.
 27. A communication device asclaimed in claim 25, wherein the digital-to-analog converter comprises:a differential pair of outputs; an array of selectable current elementsconfigured to provide respective reference currents to either output ofthe differential pair of outputs; a selector configured to select,dependent on the digital fine control signal, which of the currentelements provide their respective reference current to the outputs; anda current controller configured to control the selected referencecurrent dependent on the digital fine control signal.
 28. Acommunication device as claimed in claim 25, wherein thedigital-to-analog converter is configured to scale the analog finecontrol signal in response to the digital fine control signal while anamplitude range of the digital fine control signal remains constant. 29.A communication device as claimed in claim 25, wherein thedigital-to-analog converter is configured to provide the analog finecontrol signal as a differential signal, and the coarse amplitudecontroller is configured to provide differential to single endedconversion of the analog fine control signal.